Computer And LCD TV Repairing

I received this TV with no life and once I plugged in nothing happened just the front red LED was blinking and therefore the TV didn't turn on!

After opening the cover I faced with three PCB circuit boards like power supply, main board and t –con board. I started with power supply all the output voltages are good.

In this model of Samsung TV, if power supply is ok but TV doesn't On, the suspect is typically EEPROM- IC 24c02n and therefore the program of this IC must are corrupted.

Since I even have the copy of this EEPROM IC, by using an EEPROM programmer it had been easy to me to overwrite the old corrupted memory.

I connected the TV to power outlet and noted the TV was as dead. From my experience totally dead equipment no sign of life are caused by power supply failure and typically repair is simply considering that power supply section has only a few components and testing one by one.

I opened the tv and that i went straight to power supply section and began scanning.

Since the tv was totally dead, I started with the safely components and searching at main fuse I noted it had been darkened inside.

the fuse died under heavy stress and because of that never replace a fuse and apply power since it'll surely blow again.

if you see a darkened fuse expect heavy short ahead So I pulled my meter in continuity mode and tested across the switching transistor (S.O.T) between the center leg(collector) and therefore the emitter and got a beep either way.

That confirmed the transistor was short, I found the surge limiter resistor was also open.

After I checked the S.O.T driver transistor 2SC3807 and located it shorted base – emitter junction.

Kindly note that whenever you discover the switching transistor shorted (S.O.T) it's almost certain this drive transistor also has suffered plenty of stress and may get replaced otherwise chances of call back are high.

After replacing the four components now it's time to check the progress thus far and so I connected the tv to power source via series lamp current limiter.

After applying power, I noted the bulb was bright then dim and this confirmed to me no more shorted component on power board.

Before connected power directly without the current limiter i made a decision to search out what exactly was the reason for this short as there was no power surge within the area and therefore the customer confirmed that the children were watching the TV when it went down.

So i made a decision to check health of all main filter capacitors using my ESR meter and that i bumped on one capacitor on the B+ line rated 4.7 uF/160 V which has an ESR reading of 0.16K (160 ohms)

Usually when a capacitor ESR has gone up the voltage on it line drops and since the B+ line is very monitored, means the feedback circuit kicks in and advice the primary source to extend the voltage (drive fast) and this caused the S.O.T transistor to overdrive and hence short causing a series of other damages to the primary side of power supply.

So just in case I did not do further investigation and located the basis reason for this problem then it means it might just a matter of your time before the tv goes down again.

It’s time to power up the TV, And it had no audio(sound)

So back again to troubleshooting now the audio circuit and this tv was using ic AN7522N.

Searched its datasheet from internet and this what I got. Confirmed VCC pin is pin1.

I did live testing and that i got zero voltage on pin1, I checked backward trying to find what might be blocking this voltage and located resistor (R465) open.

I checked if the I.C was shorted, and was found to be okay.

I replaced the open resistor and applied power and noted the voltage 12 volts to the audio circuit is stable, this confirmed there's no short on it line and at last I re-assembled everything and after applying power I noticed the audio was now back.

LCD TV 24″ came to repair with the complaint of red light problem. Red light means stand by power fault. It also mean no display until you've got sort the matter out. Stand by problem usually are often caused by bad power supply or The mother board. If Power supply is supplying good 5 volt DC then the fault is within the main board.

If power supply have only few volts ( 2 to three volt) this might mean power DC rail filter capacitor could have fault. So during this case, I got 5 volt DC output from power supply and that i measured it at the output connector pin 4 and 5 The DC 5 volt goes to the 2 boards (tuner and main board):
The easiest point to see within the mainboard is that the SMD transformer IC. check out the part number and see what is going to be the output voltage.

In my case, it had been the three .3 volt transformer IC. it's the input 5 volt but doesn't have 3.3 volt. it had been lowered than 3.3 volt!
Because of output voltage was lower that mean this particular IC could have problem. you furthermore may got to know that sometimes the lowered voltage might be caused by a leaky component along the three .3 volt line.
I replaced the IC and did a check on the output voltage. It was 3.3 volt! So this suggests there's nothing leaky within the 3.3 volt rail.
I lifted panel and saw the display if working or not. I replace everything and powered it on.
Conclusion- If after fixing the new 3.3 volt transformer IC and therefore the output voltage remains low then i want to see on the components that's connected to the three .3 volt power rail. It might be a shorted cap, leaky IC and etc.